1. Field of the Invention
The present invention relates generally to a steering system for causing an automobile to steer in accordance with the operation of a driver.
2. Description of the Related Art
The steering of the automobile is realized by transmitting the operation of steering operating means, for example, the rotation of the steering wheel arranged within the cabin to a steering mechanism which is disposed externally the cabin and operates the wheels for steering use (generally the front wheels).
Various types such as ball screw type, rack and pinion type or the like are put into practical use as an steering mechanism for the automobile. For example, a rack and pinion type steering mechanism, which is adapted to transmit through tie rods to knuckle arms provided on the front right and left wheels, the sliding movement along the length direction of the rack shaft extended in the right and left directions in the front portion of the automobile body, interlocks with the pinion engaged with the tip end of the rotation shaft (steering column) of the steering wheel extended out of the cabin with the rack teeth formed halfway in the rack shaft. Such steering mechanism converts the rotation of the steering wheels into the sliding, in the length direction, of the rack shaft to effect the steering operation in accordance with the rotating operation of the steering wheels.
Also, in recent years, in order to reduce steering burden of the driver, a power steering system is put into wider use. The powder steering system has an actuator for steering auxiliary use such as hydraulic cylinder, electric motor or the like halfway provided in the steering mechanism. The operation of the steering mechanism in accordance with the rotation of the steering wheel is assisted by the driving operation of the actuator in accordance with the detected results of the operating force applied to the steering wheel for steering operation.
But such a conventional steering system as described above has problems such as the mechanical coupling between the steering wheel as the steering operating means and the steering mechanism as the steering means is required in spite of whether or not it is constructed as a power steering system, and the arrangement position of the steering wheel within the cabin is restricted in a position at which coupling with the steering mechanism outside of the cabin can be realized. Also, complicated coupling construction is required for realizing the coupling therebetween even if the steering wheel is arranged in a position at which coupling with respect to the steering mechanism can be realized, thus resulting in factors interfering with light weight automobile, simpler assembling steps.
A steering system for automobiles designed to solve such problems is disclosed in Japanese Utility Model Publication No. 2-29017 (1990). In the steering system, the steering wheel is disengaged from the steering mechanism, and also, an electric motor is arranged as an actuator for steering use halfway in the steering mechanism as same in the actuator for assisting steering of the power steering system. The electric motor is driven in accordance with the detected results of the operating direction and operation amount of the steering wheel to effect the steering operation in accordance with the operation of the steering wheel.
The steering wheel disengaged from the steering mechanism has a reaction force actuator having a electric motor and a gear mechanism. The reaction force actuator applies the reaction force, which becomes larger or smaller in accordance with the vehicle speed and the steering angle, to the steering wheel by driving of the electric motor in accordance with the detected results of the vehicle speed and the steering angle. The steering operation is adapted to be effected with the same feelings as those of the general steering system (coupling type steering system) mechanically coupled between the steering wheel and the steering mechanism by increasing or decreasing the motor current of the electric motor for steering use in accordance with the detected results, namely, by increasing or decreasing the steering force generated by the electric motor on the basis of the detection result of the operation force to be applied to the steering wheel against the reaction force.
The conventional separation type steering system constructed above has following advantages. The steering system is useful for the development of the automobile technique in the future, as realization of the new steering means, such as a lever, a pedal, a joy stick and so on, replacing the conventional steering wheel, and realization of the automatic driving system following the driving information such as detection of the guiding marks on the road surface, reception of the information from satellites or the like, in addition to the above described objects such as increase of flexibility of the arrangement of the steering wheel, light weight automobile or the like. However, above conventional separation type steering system has following problems.
The operation force to be applied to the steering wheel for steering operation is necessary for changing the direction of the wheels for steering use against the reaction force from the road surface, and is known to become usually larger accompanied by the reduction in the vehicle speed, also larger accompanied by the increase in the steering angle. Accordingly, it is natural to feel lighter at the smaller steering operation at higher speed running of the steering wheel by a driver, and conversely, heavier at the larger steering operation at lower speed running by the direction action of such reaction force as described above in the case of the operation of the steering wheel in the general coupling type steering system.
On the other hand, in the separation type steering system disclosed in the above described Japanese Utility Model Publication No. 2-29017 (1990), the reaction force actuator provided on the steering wheel is adapted to apply the reaction force which becomes larger or smaller in accordance with one or both of the higher and lower vehicle speeds and the larger or smaller steering angles. But such a reaction force applying way does not correspond to the conventional general coupling type steering system. Therefore, unnatural operation feelings are given to the driver. The same steering operation as in the general steering system is not realized because of the generated force of the electric motor for steering use to be driven in accordance with the operation of the steering wheel against the reaction force.
Further, the reaction force from the road surface which is applied to the wheel for steering use does not increase or decrease simply by the vehicle speed and the steering angle. For example, even under the same conditions in the vehicle speed and the steering angle, the reaction force acting upon the wheel at running on the snow road becomes smaller than at running on the paved road, and reversibly, the reaction force acting upon the wheels at running on the gravel road becomes larger than at running operation on the paved road. In the separation type steering system disposed in the above described Japanese Utility Model Publication No. 2-29017 (1990), the same reaction force as that at running on the paved road is applied to the steering wheel even at running on the snow road or the gravel road. The steering wheel is effected against the reaction force so that there is a possibility of effecting an excessive steering operation by the driver at running on the snow road, thus causing slipping accidents to happen. Also, reversibly the driver may effect the excessively small steering operation at running on the gravel road so that the curve may not be turned by the driver.
On the other hand, there are request that the driver desires to reduce the operation force at steering and also, to reduce the operation amount to solve the bother of the steering operation when complicated steering operations onto both the right and left sides are necessary for the large reaction force in such a running condition at an extremely slow speed or at a substantially stop condition as garaging the automobile, bringing the automobile near to crosswise direction or the like. To meet such request, Japanese Patent Application Laid-Open No. 62-120272 (1987), Japanese Patent Application Laid-Open No. 63-291770 (1988) disclose that a change speed mechanism is arranged halfway in the steering column with the coupling type steering system as an object, and the operation amount of the steering mechanism with respect to the operating amount of the steering wheel is adapted to change to become larger at running in low speed or at stop time than at running in operation at the high speed by changes in the speed change ratio of the speed change mechanism in accordance with the detected results of the vehicle speed.
In such construction, heavy and massive components such as actuator or the like for speed changing are required for the speed change mechanism. Thus, it is difficult in reality to provide such speed change mechanism as described above halfway in the steering coelum. Even if the components are possible to arrange, it is unsuitable to go against the light weight automobile.